The present invention relates to a liquid crystal composition and a liquid crystal shutter arrangement containing such a composition, for example a liquid crystal display device including a multiplicity of liquid crystal shutters.
Liquid crystal devices including ferroelectric liquid crystal compositions are the subject of active research interest. It is particularly required for there to be a good contrast ratio between the switched states of the liquid crystal device. In this regard, the bistable nature of these devices, and the switching properties and memory angle (.theta..sub.m) thereof are important. The memory angle (.theta..sub.m) is half the angular difference between the optic axes of the two latched orientation states of the liquid crystal molecules and is optimally 22.5.degree., which is difficult to achieve in practice. A target value of greater than 15.degree. is advantageous. The required alignment of the molecules within the liquid crystal layer results from appropriate treatment of substrates on opposite sides of such layer. The liquid crystal layer is formed by introducing the liquid crystal composition at an elevated temperature so as to cause it to be in the isotropic, free flowing, phase into a narrow gap (the cell gap) between the substrates. At this stage, the liquid crystal composition is at a temperature somewhat higher than its operating temperature.
The liquid crystal composition is then cooled in a controlled manner until an optically active tilted smectic phase is achieved, typically the smectic C phase, SmC*, (the * denotes chirality or optical activity). In order for the liquid crystal composition to have the required advantageous properties, it is desired for it to pass from the isotropic phase (I) through chiral nematic (N*) and smectic A (SmA) phases before reaching the operative smectic C* phase. This is referred to as an I-N*-SmA-SmC* phase sequence. These phases must be of acceptable width (greater than about 2.degree. C. wide).
Liquid crystal compositions commonly comprise a host material such as those based on mixtures of two or more fluorinated phenylpyrimidinyl compounds exhibiting or capable of exhibiting a smectic C phase and, optionally, other materials (phase sequence enhancers) which may be included to modify the properties of the host material, for example to impart chirality if the host material is not inherently chiral, or to induce or enhance formation of certain phases so as to result in the required phase sequence.
U.S. Pat. No. 5,232,624 and EP-A-0332025 disclose a wide variety of chiral 1,2-difluorobenzene derivatives for imparting chirality in achiral smectic liquid crystal host mixtures. In one particular example, there is described the addition of 10% of a chiral dopant, ethyl 2-[4-(4'-nonyloxy-2',3'-difluorobiphenyl4-yl)phenoxy] propionate, to a host mixture which consists mostly of unfluorinated phenylpyrimidine compounds and phenyl-substituted thiadiazoles but which contains minor proportions of two difluorinated phenylpyrimidines and a difluorinated biphenylpyrimidine.